The specific ECG characteristics of metabolic acidosis have not been satisfactorily defined in man. We addressed this question by prospectively studying 14 consecutive patients admitted with metabolic acidosis and without hyperkalemia. Arterial blood gas analysis and serum potassium determinations were performed during acidosis and after its correction. ECGs were recorded at the same times. Mean pHa increased from 7.11 +/- 0.05 to 7.40 +/- 0.01 (p less than .001) in response to an increase in total CO2 content from 6.8 +/- 1.4 to 21.2 +/- 0.7 mmol/L (p less than .001). Serum potassium concentration decreased only slightly after correction of the acidosis from 4.2 +/- 0.1 to 3.9 +/- 0.14 mmol/L (p less than .05). T waves were taller during acidosis than after correction (0.68 +/- 0.1 vs. 0.28 +/- 0.04 mV, p less than .001 and 0.64 +/- 0.08 vs. 0.38 +/- 0.04, p less than .005, in precordial leads V2 and V3, respectively). Moreover, the amplitude of T waves in V2 was correlated positively with the arterial concentration of H+ (r = .786, p less than .001) and negatively with the arterial total CO2 content (r = -.71, p less than .005). In addition to their augmented amplitude, T waves were peaked and symmetrical with a narrow base ("tent-shaped"). Thus, metabolic acidosis may be accompanied by an increase in the amplitude of T waves, even in the absence of hyperkalemia.